I have played around with my BZLS as a head amp. What you have to realize is that if you only have the output pots then this set up will not work well since the pot impedance is too high for proper volume management into headphones.If you also have the input pots then that is no problem since you can turn the out pots all the way up and use the input pots for volume control.
If you also want to experiment with balanced operation fully into headphones then you might need a repacement head cord that you can modify with a four pin connector so that you have +- left and +-right.
The BZLS has a great sound driving headphones and I am currently using it as a headphone amp although modified to run on batteries at+- 12V.
The stock unit would be difficult to modify with a headphone jack connected in parallel to the outputs because you would have to short the two negative outputs pemanently.The solution is to use a small four pole switch that would be wired appropriately to switch between normal balanced line out and headphone out.This is workable but ultimately you would have to take the output before the output 10mf caps so you don't lose any low frequencies.
All this sounds complicated but in the end may simplify your set up by having your pre and head amp in component.

When I cranck the volume up, it is still quiet! BTW, I have Bride of Zen, not bzls - so do I still need to take the output before the output 10mf caps? Also, I hooked it up onto a discman, not some line player - maybe that's why. Also, it was connected to a headphone jack to my Marantz cd63 - then it played good, but that's allready an headphone amp!! Can you help me a bit? Thanks!!

Unfortunately the BOZ has an output pot which , because of its high impedance , will not allow it to output enough voltage if connected to a low impedance load like a few hundred ohm headphones. Do not attempt to take any output from before the caps since there is a lot of DC which will blow your headphones.
On the BZLS you can get away with not using caps if you use just the balanced outputs and not output to ground since the balanced +- outputs only have relatively little DC offset between them.
As mentioned in previous postings perhaps the best solution is to build a separate head amp which is very easy to do such as the c-moy or even meta-42.These will not work with the power supply of the BOZ since they can only handle about +- 15V maximum. However they work well with batteries or even a simple (linear please) wall wart.